The present invention relates to a flexible fastening device for attaching a longitudinal element to an internal or external osseous anchor device.
In particular, the present invention relates to a link system in particular for a typically cylindrical fastening bar comprising an osseous anchor device consisting of an intra-osseous anchor part separated from an extra-osseous part associated to a tightening element. The present invention generally relates to a fastening device designed in particular to attach said fastening bar to the anchor element.
In common practice, the connection between an osseous anchor device and an overall longitudinally shaped piece is realised by various devices such as articulated or non-articulated clamps, clamping rings or half-rings, open or close brackets, assembly nuts provided with various tightening means.
Several known systems enable the blocking of the fastening bar with relation to the osseous anchor device, but with the bar maintained in one direction and in one fixed position with relation to the anchor device. For instance, patent FR-A-2 657 775 describes in particular a spinal bar link system with the object of linking the vertebrae comprising an osseous screw anchor with a threaded head receiving a fastening device engaging the threaded head enabling the blocking of the fastening bar between said fastening device and the cone-shaped part of a locking nut engaged on the threaded head.
U.S. Pat. No. 5,716,355 describes in particular an articulated fastening device aimed at interlocking a longitudinal element of an essentially elongated shape and an osseous anchor device, which is either internal, in particular vertebral, or external, in accordance with the preamble of Claim 1. Said fastening device comprises first and second connection parts designed to receive the extra-osseous part of the anchor device. In addition, said fastening device comprises an intermediate part joining the first and second connection parts designed to fasten the longitudinal element.
Said system enables the adjustment to a certain degree of the angle formed by the axes of the bar and of the anchor device respectively within a specified plan, but after tightening it remains in a fixed and relatively rigid position and it does not feature any flexibility via a shock absorber or spring effect.
The various known fastening devices have the following disadvantages in common:
Absence of flexibility: no shock absorber effect, no spring effect.
Significant loss of tightening due to curved shape of longitudinal piece.
Embrittlement of longitudinal piece due to concentration of tightening pressure.
Lack of adaptability to a naturally irregular osseous geometry.
Decrease of tightening effect as distance between the anchor point and the longitudinal piece increases.
It is desirable to design a fastening device able to increase the variability of angulation of the longitudinal connecting piece with relation to the anchor device while still maintaining the fastening or blocking of the two pieces which are to be joined.
Desirable characteristics for a fastening device of this type are the variability in all directions of the relative direction of the two pieces which are to be interlocked; the possibility of obtaining a higher degree of flexibility and return movement due to a spring effect; maintaining of the fastening of the longitudinal piece even if the element is deformed, longitudinally curved or exhibits an irregular surface; adaptability to an irregular osseous geometry; maintaining of the fastening when the distance between the anchor device and the longitudinal piece increases.
Previously known devices of this type have been proven to be deficient in one respect or the other in as far as obtaining the desired characteristics by means of a simple structure which is easy to assemble and use is concerned.
One object of the present invention is to provide a flexible anchor device enabling the desired characteristics to be obtained but without the need for complicated and costly instrumentation.
A further object of the present invention is to design a flexible anchor device enabling a variation in the direction of the longitudinal fastening piece with relation to the axis of the anchor device to be obtained while improving in particular the tightening of the two pieces to be joined and hence the overall blocking.
A further object of the present invention is to provide an anchor device displaying axial, radial and rotational flexibility while enabling the translational and rotational blocking of the longitudinal element even if the element is deformed, longitudinally curved or is exhibiting an irregular surface.
In accordance with the invention, these objects and others are achieved by a new flexible fastening device displaying the characteristics of the characterising features of Claim 1.
The invention is characterised in that the flexible fastening device consists of a strand out of elastic material or a material in the process of phase transformation and in that the intermediate part of said strand comprises at least one spiral coil adapted to enclose the longitudinal element, such that the bringing closer of the first and second connection parts to the strand by means of the tightening device decreases the diameter of the spiral coil such as to ensure the translational and rotational blocking of the longitudinal element.
In general terms, the present invention, which will subsequently be described in more detail, is based upon the reduction of the internal diameter formed by a metallic strand spirally coiled with jointed turns around a typically cylindrical piece, by the relative rotation of its extremities such that it accentuates the coil initially at rest.
In theory, the developed force is proportional to the number of turns and the angular displacement of the extremities.
In practice, the friction between the longitudinal piece and the inside of the spiral coil as well as the angular displacement necessary for obtaining the fastening increase with the number of coils, which leads to a preference of their number of between 1 and 3 turns. This principle has the advantage of being tolerant towards surface or geometry irregularities, such as curving or bending exhibited by the longitudinal piece which is to be immobilised. According to the invention the fastening device exhibits axial, radial and rotational flexibility on the level of both the intermediate part and the two connection parts of the strand.
A further advantage of this aspect of the invention is that the axial, radial and rotational flexibility of the spiral coils can be selected according to the shape or size of the strand and is thus adjustable.
According to an advantageous embodiment of the invention, the metallic strand made of an elastic material or even a material in the process of phase transformation (so-called memory metal) is spirally coiled around the longitudinal element to be held, for at least one turn in one direction, a first loop or half-loop is drawn before spirally coiling in an identical, preferably symmetrically manner, in the opposite direction around the longitudinal element to be maintained.
The extremities of said strand pass on either side of the anchor device and can each end in a loop, or be advantageously joined by a sliding piece, around the extra-osseous part receiving the fastening means of the anchor device.
In order to improve the tightening of the longitudinal element while maintaining the desired overall degree of flexibility, the bringing closer by the tightening means results in an angular displacement of the first loop or half-loop towards the loops or the sliding piece, which reduces the internal diameter of the spiral coils of said flexible fastening device around the longitudinal element so as to block the latter in both a translational and rotational manner, even if said element is deformed, longitudinally curved or is exhibiting an irregular surface.
Further characteristics and advantages shall emerge from the description of several embodiments of the device according to the invention. A preferred embodiment of the present invention will be described by way of example hereinafter with reference to the figures, in which: